Crystallographic analyses have been successfully initiated for 4 proteins that undergo functionally important structural changes, which are signaled by changes in absorbance spectra. For the 6 electron reduction enzyme, sulfite reductase, the active hemoprotein subunit has been crystallized, and data collected on native crystals (2.4 angstrom resolution) and two derivatives. The proposed work involves determination of the chain tracing and atomic structure of the active hemoprotein subunit in oxidized, reduced and sulfite-bound conformations, and analysis of the structural implications for the enzyme mechanism. This will provide the first structural data on a protein containing both a siroheme and an Fe4S4 cluster and will serve as a model for other multi-center multiple electron transfer enzymes, such as cytochrome oxidase, nitrogenase, and nitrite reductase. No structures are known for Root-effect hemoglobins (Hb); the extreme effect of pH on the oxygen affinity of these Hbs allows fish to pump oxygen into the swim bladder against 100 atmospheres of oxygen pressure. Crystals have been obtained for the R and T state conformations of Leiostomas xanthurus Root-effect Hb. For the R-state conformation, 4 angstrom maps have been calculated and native data collected to 2.5 angstrom resolution. The proposed work involves the determination of both R and T-state conformations and an analysis of the structural basis for the extreme pH dependence of O2 binding. Crystals of a water-soluble photosensing protein ectorhodopsin have been grown which diffract to at least 1.5 angstrom resolution, and 2.5 angstrom resolution native data has been collected. The crystals can be bleached by visible light, but are not bleached by x-rays. The structures of both bleached and unbleached forms of ectorhodopsin will be determined and the nature of photo-induced conformational change deduced. Crystals have been obtained for Photobacter leiognathi Cu.Zn superoxide dismutase (SOD), which shares little sequence homology with other SODs and may have resulted from a gene transfer from the eucaryotic host fish. The proposed work will involve space group determination, data collection, and structure determination for oxidized and reduced SOD. Taken together, these chromatically active proteins offer the opportunity to use x-ray crystallographic information to evaluate general mechanisms for protein conformational change in 4 different systems. There are protein structures adapted for 1) multiple electron transfer (sulfite reductase). 2) light-induced conformational change (ectorhodopsin). 3) extreme conformational control of substrate binding affinity (Root-effect Hb), and 4) extremely rapid interactions with substrate, coupled to redox changes between 2 active-site conformations (SOD). Thus, this proposal seeks to elucidate factors controlling stability versus change in the conformational stages of proteins, which are basic to numerous biological processes including electron transfer, metabolic reactions, oxygen transport, photoactivation, antigenicity, and catalysis.